Tunable chromatic dispersion, dispersion slope, and polarization mode dispersion compensator utilizing a virtually imaged phased array

ABSTRACT

The present invention provides a dispersion compensator which utilizes a Virtually Imaged Phased Array (VIPA), gratings, and birefringent wedges to moderate chromatic dispersion, dispersion slope and polarization mode dispersion. The dispersion compensator in accordance with the present invention propagates the composite optical signal in a forward direction; separates the wavelengths in the band of wavelengths in each of the plurality of channels, where each of the wavelengths in the band is spatially distinguishable from the other wavelengths in the band; spatially separates each band of wavelengths in the plurality of channels; spatially separates each wavelength of each separated band of wavelengths into a plurality of polarized rays; and reflects the plurality of polarized rays toward a return direction, where dispersion is added to the reflected plurality of polarized rays such that the unwanted chromatic dispersion, dispersion slope, and PMD are compensated. The dispersion compensator provides simultaneous tunable compensation of these various dispersions utilizing a single apparatus. A system which utilizes the compensator is thus cost effective to manufacture.

FIELD OF THE INVENTION

The present invention relates to chromatic dispersion, dispersion slope,and polarization mode dispersion compensation, and more particularly todispersion compensation accumulated in a wavelength division multiplexedoptical fiber transmission line.

BACKGROUND OF THE INVENTION

Fiber optic networks are becoming increasingly popular for datatransmission because of their high speed and high capacity capabilities.Wavelength division multiplexing is used in such fiber opticcommunication systems to transfer a relatively large amount of data at ahigh speed. In wavelength division multiplexing, multipleinformation-carrying signals, each signal comprising light of a specificrestricted wavelength range, may be transmitted along the same opticalfiber.

In this specification, these individual information-carrying lights arereferred to as either “signals” or “channels.” The totality of multiplecombined signals in a wavelength-division multiplexed optical fiber,optical line or optical system, wherein each signal is of a differentwavelength range, is herein referred to as a “composite optical signal.”

One common and well-known problem in the transmission of optical signalsis chromatic dispersion of the optical signal. Chromatic dispersionrefers to the effect wherein the channels comprising an optical signaltravel through an optic fiber at different speeds, e.g., longerwavelengths travel faster than shorter wavelengths. This is a particularproblem that becomes more acute for data transmission speeds higher than2.5 gigabytes per second. The resulting pulses of the signal will bestretched, will possibly overlap, and will cause increased difficultyfor optical receivers to distinguish where one pulse begins and anotherends. This effect seriously compromises the integrity of the signal.Therefore, for a fiber optic communication system to provide a hightransmission capacity, the system must compensate for chromaticdispersion. The exact value of the chromatic dispersion produced in achannel of a wavelength-division multiplexed fiber optic communicationssystem depends upon several factors, including the type of fiber and thewavelength of the channel.

For dense wavelength division multiplexer (DWDM) systems or for WDM orDWDM systems with a wide wavelength spacing between the shortest andlongest wavelength channels, the common approach is to allow chromaticdispersion to accumulate within spans of fiber and to compensate fordispersion at the ends of spans through the use of in-line dispersioncompensator apparatuses.

A second common and well-known problem in the transmission of opticalsignals is polarization mode dispersion (PMD). PMD is the phenomenon bywhich differently polarized components, or sub-signals, comprising anoptical signal propagate with different speeds or, alternatively,propagate along differing-length optical paths. This duality of speedsor paths can also cause unacceptable broadening of the digital pulsescomprising a signal that increases in severity with increasingtransmission speed. The maximum acceptable PMD-inducted optical pathlength difference is the cumulative result of all PMD effects in all theoptical elements through which a signal propagates, including fiber andnon-fiber optical components. Although the PMD broadening of opticalfiber increases as the square root of fiber length, the PMD broadeningcaused by birefringent components is linearly related to the cumulativeoptical path difference of all such components.

The chromatic dispersion characteristics of optical fibers are notconstant but depend upon wavelength, as illustrated in FIG. 1, whichpresents graphs of Group Velocity Dispersion, D, against wavelength, fortypical examples of three commonly used fiber types. In FIG. 1, thequantity D (ps-km⁻¹-nm⁻¹) is defined by the relationship of Eq. 1$\begin{matrix}{D = {{\frac{}{\lambda}\left( \frac{1}{v_{g}} \right)} = {\frac{1}{L}\left( \frac{\tau_{g}}{\lambda} \right)}}} & (1)\end{matrix}$

in which λ is the channel wavelength (nm), ν_(g) is the group velocity(km/ps), τ_(g) is the group delay time (ps), and L is the fiber length(km). If ν_(g) decreases with increasing wavelength (i.e., longer or“red” wavelengths travel slower than relatively shorter or “blue”wavelengths) then D is positive, otherwise D is negative. Because allthree fiber types illustrated in FIG. 1 are deployed intelecommunications systems, the requirements for dispersion compensatorsvary widely. Furthermore, because of the existence of non-zerodispersion slope, S, a constant level of dispersion compensation doesnot accurately negate the dispersion of all channels. This inaccuracycan become a significant problem for high-speed data propagation, longspan distances and/or wide distances between the shortest and longestwavelength channels.

Conventional apparatuses for dispersion compensation include dispersioncompensation fiber, chirped fiber Bragg gratings coupled to opticalcirculators, and conventional diffraction gratings disposed assequential pairs.

A dispersion compensation fiber, which is used in-line within a fibercommunications system, has a special cross-section index profile so asto provide chromatic dispersion that is opposite to that of ordinaryfiber within the system. The summation of the two opposite types ofdispersion negates the chromatic dispersion of the system. However,dispersion compensation fiber is expensive to manufacture, has arelatively large optical attenuation, and must be relatively long tosufficiently compensate for chromatic dispersion.

A chirped fiber Bragg grating is a special fiber with spatiallymodulated refractive index that is designed so that longer (shorter)wavelength components are reflected at a farther distance along thechirped fiber Bragg grating than are the shorter (longer) wavelengthcomponents. A chirped fiber Bragg grating of this sort is generallycoupled to a fiber communications system through an optical circulator.By causing certain wavelength components to travel longer distances thanother wavelength components, a controlled delay is added to thosecomponents and opposite dispersion can be added to a pulse. However, achirped fiber Bragg grating has a very narrow bandwidth for reflectingpulses, and therefore cannot provide a wavelength band sufficient tocompensate for light including many wavelengths, such as a wavelengthdivision multiplexed light. A number of chirped fiber Bragg gratings maybe cascaded for wavelength multiplexed signals, but this results in anexpensive system. Furthermore, fiber Bragg gratings generally do notcompensate polarization mode dispersion.

A conventional diffraction grating has the property of outputtingdifferent wavelengths at different angles. By using a pair of gratingsin a coupled spatial arrangement, this property can be used tocompensate chromatic dispersion in a fiber communications system. Insuch a spatial grating pair arrangement, lights of different wavelengthsare diffracted from a first grating at different angles. These lightsare then input to a second grating that diffracts them a second time soas to set their pathways parallel to one another. Because the differentlights travel with different angles between the two gratings, certainwavelength components are made to travel longer distances than otherwavelength components. Chromatic dispersion is produced in the spatialgrating pair arrangement because the wavelength components that travelthe longer distances incur time delays relative to those that travel theshorter distances. This grating-produced chromatic dispersion can bemade to be opposite to that of the fiber communications system, therebycompensating the chromatic dispersion within the system. However, apractical spatial grating pair arrangement cannot provide a large enoughdispersion to compensate for the relatively large amount of chromaticdispersion occurring in a fiber optic communication system. Morespecifically, the angular dispersion produced by a diffraction gratingis usually extremely small, and is typically approximately 0.05degrees/nm. Therefore, to compensate for chromatic dispersion occurringin a fiber optic communication system, the two gratings of a spatialgrating pair would have to be separated by a very large distance,thereby making such a spatial grating pair arrangement impractical.

Accordingly, there exists a need for an improved chromatic dispersion,dispersion slope, and polarization mode dispersion (PMD) compensator.The improved compensator should produce an adjustable chromaticdispersion and be readily adapted to provide either positive or negativechromatic dispersion, which can provide non-uniform dispersioncompensation so as to compensate for fiber dispersion slope, and canalso compensate for polarization mode dispersion. The present inventionaddresses such a need.

SUMMARY OF THE INVENTION

The present invention provides a dispersion compensator which utilizes aVirtually Imaged Phased Array (VIPA), gratings, and birefringent wedgesto moderate chromatic dispersion, dispersion slope and polarzation modedispersion. The dispersion compensator in accordance with the presentinvention propagates the composite optical signal in a forwarddirection; separates the wavelengths in the band of wavelengths in eachof the plurality of channels, where each of the wavelengths in the bandis spatially distinguishable from the other wavelengths in the band;spatially separates each band of wavelengths in the plurality ofchannels; spatially separates each wavelength of each separated band ofwavelengths into a plurality of polarized rays; and reflects theplurality of polarized rays toward a return direction, where dispersionis added to the reflected plurality of polarized rays such that theunwanted chromatic dispersion, dispersion slope, and PMD arecompensated. The dispersion compensator provides simultaneous tunablecompensation of these various dispersions utilizing a single apparatus.A system which utilizes the compensator is thus cost effective tomanufacture.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph of the typical dispersion characteristics of threecommon commercially available optical fiber types, plotted againstwavelength.

FIG. 2 illustrates a Virtually Imaged Phased Array (VIPA) utilized inthe preferred embodiments of a dispersion compensator in accordance withthe present invention.

FIG. 3 illustrates in more detail the light path through and operationof the VIPA of FIG. 2.

FIG. 4 is a diagram illustrating an apparatus which uses a VIPA and alight returning device to produce chromatic dispersion.

FIG. 5 is a more detailed diagram illustrating the operation of theapparatus in FIG. 4.

FIGS. 6a and 6 b are diagrams illustrating side views of an apparatuswhich uses a VIPA together with a curved light reflecting apparatus soas to provide, respectively, negative and positive chromatic dispersionto light.

FIGS. 7a and 7 b illustrate a top-view and side-view, respectively, of afirst preferred embodiment of a dispersion compensator in accordancewith the present invention.

FIG. 7c is a perspective view showing the locations of the focusedwavelengths of the various channels upon the mirrors of the firstpreferred embodiment of the compensator in accordance with the presentinvention.

FIGS. 8a and 8 b illustrate a top-view and side-view, respectively, of asecond preferred embodiment of a dispersion compensator in accordancewith the present invention.

FIGS. 9a and 9 b illustrate a top-view and side-view, respectively, of athird preferred embodiment of a dispersion compensator in accordancewith the present invention.

FIGS. 10a and 10 b illustrate a top-view and side-view, respectively, ofa fourth preferred embodiment of a dispersion compensator in accordancewith the present invention.

FIG. 11 illustrates a preferred embodiment of a system which utilizes adispersion compensator in accordance with the present invention.

DETAILED DESCRIPTION

The present invention provides an improved chromatic dispersion,dispersion slope, and polarization mode dispersion (PMD) compensator.The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe preferred embodiment will be readily apparent to those skilled inthe art and the generic principles herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown but is to be accorded the widest scopeconsistent with the principles and features described herein.

The present invention provides a dispersion compensator which comprisesa Virtually Imaged Phased Array (VIPA) optically coupled on a first sideto an optical communications system, and optically coupled on a secondside to one or more transmission-type diffraction gratings and a mirrorand one or more birefringent wedges. The VIPA assists in producingchromatic dispersion compensation, the diffraction grating assists inproducing dispersion slope compensation, and the birefringent wedgeassists in compensation for PMD.

FIG. 2 illustrates a VIPA utilized in the preferred embodiments of thedispersion and dispersion compensator in accordance with the presentinvention. The VIPA 76 is disclosed in U.S. Pat. No. 5,930,045,incorporated herein by reference. The VIPA 76 is preferably made of athin plate of glass. An input light 77 is focused into a line 78 with alens 80, such as a semi-cylindrical lens, so that input light 77 travelsinto VIPA 76. Line 78 is hereinafter referred to as “focal line”. Inputlight 77 radially propagates from focal line 78 to be received insideVIPA 76. The VIPA 76 then outputs a luminous flux 82 of collimatedlight, where the output angle of luminous flux 82 varies as thewavelength of input light 77 changes. For example, when input light 77is at a wavelength λ₁, VIPA 76 outputs a luminous flux 82 a atwavelength λ₁ in a specific direction. When input light 77 is at awavelength λ₂, VIPA 76 outputs a luminous flux 82 b at wavelength λ₂ ina different direction. Therefore, VIPA 76 produces luminous fluxes 82 aand 82 b that are spatially distinguishable from each other.

FIG. 3 illustrates in more detail the VIPA 76 and light paths thereinand therethrough. The VIPA 76 includes a plate 320 made of, for example,glass, and having reflecting films 322 and 324 thereon. Reflecting film322 preferably has a reflectance of approximately 95% or higher, butless than 100%. Reflecting film 324 preferably has a reflectance ofapproximately 100%. A radiation window 326 is formed on plate 320 andpreferably has a reflectance of approximately 0% reflectance.

Input light 77 is focused into focal line 78 by lens 80 throughradiation window 326, to subsequently undergo multiple reflectionbetween reflecting films 322 and 324. Focal line 78 is preferably on thesurface of plate 320 to which reflecting film 322 is applied. Thus,focal line 78 is essentially line focused onto reflecting film 322through radiation window 326. The width of focal line 78 can be referredto as the “beam waist” of input light 77 as focused by lens 80. Thus,the VIPA 76 focuses the beam waist of input light 77 onto the farsurface (that is, the surface having reflecting film 322 thereon) ofplate 320. By focusing the beam waist on the far surface of plate 320,the VIPA 76 reduces the possibility of overlap between (i) the area ofradiation window 326 on the surface of plate 320 covered by input light77 as it travels through radiation window 326 and (ii) the area onreflecting film 324 covered by input light 77 when input light 77 isreflected for the first time by reflecting film 324. It is desirable toreduce such overlap to ensure proper operation of the VIPA 76.

In FIG. 3, an optical axis 332 of input light 77 has a small tilt angleθ with respect to a line 340 perpendicular to the plane of plate 320.Upon the first reflection off of reflecting film 322, 5% of the lightpasses through reflecting film 322 and diverges after the beam waist,and 95% of the light is reflected towards reflecting film 324. Afterbeing reflected by reflecting film 324 for the first time, the lightagain hits reflecting film 322 but is displaced by an amount d. Then, 5%of the light passes through reflecting film 322. In a similar manner,the light is split into many paths with a constant separation d. Thebeam shape in each path forms so that the light diverges from virtualimages 334 of the beam waist. Virtual images 334 are located withconstant spacing 2t along a line 340 that is normal to plate 320, wheret is the thickness of plate 320. The positions of the beam waists invirtual images 334 are self-aligned, and there is no need to adjustindividual positions. The lights diverging from virtual images 334interfere with one other and form collimated light 336 which propagatesin a direction that changes in accordance with the wavelength of inputlight 77.

The spacing of light paths is d=2t sin θ, and the difference in the pathlengths between adjacent beams is 2t cos θ. The angular dispersion ofthe VIPA 76 is proportional to the ratio of these two numbers, which iscot θ. As a result, a VIPA 76 produces a significantly large angulardispersion.

Reflecting surfaces 322 and 324 are in parallel with each other andspaced by the thickness t of plate 320 and are typically reflectingfilms deposited on plate 320. As previously described, reflectingsurface 324 has a reflectance of approximately 100%, except in radiationwindow 326, and reflecting surface 322 has a reflectance ofapproximately 95% or higher. Therefore, reflecting surface 322 has atransmittance of approximately 5% or less so that approximately 5% orless of light incident on reflecting surface 322 will be transmittedtherethrough and approximately 95% or more of the light will bereflected. The reflectances of reflecting surfaces 322 and 324 caneasily be changed in accordance with the specific VIPA application.However, generally, reflecting surface 322 should have a reflectancewhich is less than 100% so that a portion of incident light can betransmitted therethrough. This reflectance need not be constant alongthe reflecting film 322.

The reflecting surface 324 has radiation window 326 thereon. Radiationwindow 326 allows light to pass therethrough, and preferably has noreflectance, or a very low reflectance. Radiation window 326 receivesinput light 77 to allow input light 77 to be received between, andreflected between, reflecting surfaces 322 and 324.

A VIPA 76 has strengthening conditions which are characteristics of thedesign of the VIPA 76. The strengthening conditions increase theinterference of the output lights so that a luminous flux is formed. Thestrengthening conditions of the VIPA are represented by the followingequation:

2t cos Φ=mλ

in which Φ indicates the propagation direction of the resulting luminousflux as measured from a line perpendicular to the surface of reflectingsurfaces 322 and 324, λ indicates the wavelength of the input light, tindicates the distance between the reflecting surfaces 322 and 324, andm indicates an integer. Therefore, if t is constant and m is assigned aspecific value, then the propagation direction Φ of the luminous fluxformed for input light having wavelength λ can be determined.

More specifically, input light 77 is radially dispersed from focal line78 through a specific angle. Therefore, input light having the samewavelength will be traveling in many different directions from focalline 78, to be reflected between reflecting surfaces 322 and 324. Thestrengthening conditions of the VIPA 76 cause light traveling in aspecific direction to be strengthened through interference of the outputlights to form a luminous flux having a direction corresponding to thewavelength of the input light. Light traveling in a different directionthan the specific direction required by the strengthening condition isweakened by the interference of the output lights.

FIG. 4 illustrates an example prior-art apparatus that uses a VIPA as anangular dispersive component to produce chromatic dispersion. Adescription of this prior-art apparatus will assist in understanding thefunctioning of the compensator 700 in accordance with the presentinvention. As illustrated in FIG. 4, a light is output from a fiber 446,collimated by a collimating lens 448 and line-focused into VIPA 440through radiation window 447 by a cylindrical lens 450. The VIPA 440then produces a collimated flight 451 which is focused by a focusinglens 452 onto a mirror 454. Mirror 454 can be a mirror portion 456formed on a substrate 458. Mirror 454 reflects the light back throughfocusing lens 452 into VIPA 440. The light then undergoes multiplereflections in VIPA 440 and is output from radiation window 447. Thelight output from radiation window 447 travels through cylindrical lens450 and collimating lens 448 and is received by fiber 446.

Therefore, light is output from VIPA 440 and reflected by mirror 454back into VIPA 440. The light reflected by mirror 454 travels throughthe path which is nearly exactly opposite in direction to the paththrough which it originally traveled. As described in greater detailherein following, different wavelength components in the light arefocused onto different positions on mirror 454, and are reflected backto VIPA 440. As a result, different wavelength components traveldifferent distances, to thereby produce chromatic dispersion.

FIG. 5 illustrates in more detail the example prior-art apparatusillustrated in FIG. 4. Assume a light having various wavelengthcomponents is received by VIPA 440. The VIPA 440 will cause theformation of virtual images 560 of beam waist 562, where each virtualimage 560 “emits” light. Focusing lens 452 focuses the differentwavelength components in a collimated light from VIPA 440 at differentpoints on mirror 454. More specifically, a longer wavelength 564 focusesat point 572, a center wavelength 566 focuses at point 570, and ashorter wavelength 568 focuses at point 574. Then, longer wavelength 564returns to a virtual image 560 that is closer to beam waist 562, ascompared to center wavelength 566. Shorter wavelength 568 returns to avirtual image 560 that is farther from beam waist 562, as compared tocenter wavelength 566. Thus, the arrangement provides for normaldispersion.

Mirror 454 is designed to reflect only light in a specific interferenceorder, and light in any other interference order should be focused outof mirror 454. More specifically, as previously described, a VIPA 440will output a collimated light. This collimated light will travel in adirection such that the optical path length difference betweensubsequent virtual images contributing to the collimated light is mλ,where m is an integer. The m^(th) order of interference is defined as anoutput light corresponding to m. Each order comprises a plurality ofwavelength components and the wavelength components of one order arerepeated in any other order. However, collimated lights at the samewavelength for different interference orders generally travel indifferent directions and are therefore focused at different positions.Thus, the mirror 454 can be made to reflect only light from a singleinterference order back into VIPA 440.

A wavelength division multiplexed light usually includes many channels,wherein each channel has a center wavelength and the center wavelengthsare usually spaced apart by a constant frequency spacing. If thethickness t between first and second surfaces 442 and 444 of VIPA 440 isset at a specific value, the arrangement will be able to simultaneouslycompensate for dispersion in each channel. The thickness t which permitssuch simultaneous dispersion compensation is such that all of thewavelength components corresponding to the center wavelengths have thesame output angle from VIPA 440 and thus the same focusing position onmirror 454. This is possible when the thickness t is set so that, foreach channel, the round-trip optical length through VIPA 440 traveled bythe wavelength component corresponding to the center wavelength is amultiple of the center wavelength of each channel, that is, t is suchthat the quantity 2 nt cos θ is an integer multiple of the centerwavelength of each channel. This amount of thickness t will hereafter bereferred to as the “WDM matching free spectral range thickness”, or “WDMmatching FSR thickness”.

Therefore, in FIG. 5, with the thickness t set to the WDM matching FSRthickness, VIPA 440 and focusing lens 452 will cause (a) the wavelengthcomponent corresponding to the center wavelength of each channel to befocused at point 570 on mirror 454, (b) the wavelength componentcorresponding to the longer wavelength component of each channel to befocused at point 572 on mirror 454, and (c) the wavelength componentcorresponding to the shorter wavelength component of each channel to befocused at point 574 on mirror 454. Therefore, VIPA 440 can be used tocompensate for chromatic dispersion in all channels of a wavelengthdivision multiplexed light. However, this prior-art VIPA-baseddispersion-compensating apparatus does not compensate for dispersionslope or PMD.

FIGS. 6a and 6 b illustrate additional example prior-art apparatuseswhich use a VIPA to provide various values of chromatic dispersion tolight. In FIGS. 6a and 6 b, there are illustrated the travel directionsof a longer wavelength 564, a center wavelength 566 and a shorterwavelength 568 of light emitted by a virtual image 560 of beam waist562. The mirror 654 and the mirror 655 are located at or near the focalpoint of focusing lens 452. In FIG. 6a, mirror 654 is a convex mirror.With a convex mirror, the beam shift is magnified relative to thatproduced by a flat mirror. Therefore, a large chromatic dispersion canbe obtained with a short lens focal length and a small amount of space.In FIG. 6b, mirror 655 is a concave mirror. With a concave mirror, thesign of the dispersion is inverted relative to that produced by a flatmirror.

With either a flat mirror 454 (FIG. 5) or a convex mirror 654 (FIG. 6a),the light of longer (“red” ) wavelengths of an optical signal travels ashorter round trip distance through the apparatus then does the light ofshorter (“blue”) wavelengths of said signal. Thus, negative chromaticdispersion is introduced into the signal. This form of apparatus isuseful for compensating accumulated positive chromatic dispersion in anoptical signal. With a concave mirror 655 (FIG. 6b), the light of “red”wavelengths of an optical signal travels a greater distance through theapparatus then does the light of “blue” wavelengths of said signal and,thus, positive chromatic dispersion is introduced into the signal. Thislatter form of apparatus is useful for compensating accumulated negativechromatic dispersion in an optical signal.

FIGS. 7a and 7 b illustrate a top-view and side-view, respectively, of afirst preferred embodiment of a dispersion compensator in accordancewith the present invention. The compensator 700 comprises a fiber 702, acollimator lens 703, a cylindrical lens 704, a VIPA 706 of which thethickness is equal to the WDM matching FSR thickness, a diffractiongrating 710, a birefringent wedge 720, a focusing lens 712, and a firstand second mirror 714 a- 714 b. A wavelength-division multiplexedcomposite signal 701 is output from fiber 702, is collimated bycollimator lens 703 and is then brought to a line focus at the beamwaist 705 of VIPA 706 by the cylindrical lens 704. The channels 707 and708 are two representative channels of the composite optical signal 701.As discussed previously, the VIPA 706 spatially disperses thewavelengths comprising each one of the channels of composite signal 701,such that rays of each wavelength emanate from the VIPA 706 along raypaths which are parallel to one another but of a different directionthan rays of any other wavelength comprising each channel. Upon passingthrough and exiting the VIPA 706, the wavelengths comprising each ofthese channels are separated and dispersed within a vertical dispersionplane. For instance, the wavelengths of first and second channels, 707and 708, respectively, are separated into relatively longer wavelengths707 a and 708 a and relatively shorter wavelengths, 707 b and 708 b,respectively, together with respective continua of interveningwavelengths. Because the thickness of the VIPA 706 is equal to the WDMmatching FSR thickness, then, immediately upon exiting the VIPA 706, thepath of wavelength 707 a overlaps that of wavelength 708 a and the pathof wavelength 707 b overlaps that of wavelength 708 b.

After exiting the VIPA 706, the wavelengths comprising each channel areseparated and dispersed from one another within a horizontal dispersionplane by the transmission grating 710. The paths of the various signalsare spatially dispersed from one another according to their respectivewavelengths. The dispersion plane of transmission grating 710 is notparallel to that of the VIPA 706, however. Instead, these two dispersionplanes are perpendicular to one another. Thus, as shown in FIG. 7a, thedispersion plane of transmission grating 710 is horizontal and, uponemerging from this grating, the wavelengths comprising the first channel707 are output along a different horizontal direction from those of thesecond channel 708. The path of the relatively longer wavelength 707 aof the first channel 707 is separated within a horizontal plane fromthat of the relatively longer wavelength 708 a of the second channel708. In similar fashion, the relatively shorter wavelengths 707 b, 708 bof each channel are spatially dispersed within a horizontal plane. Thesetwice dispersed wavelengths of the plurality of channels then passthrough the birefringent wedge 720 which spatially separates each suchwavelength into a first and second rays of mutually orthogonalpolarizations. The less-deflected horizontally polarized rays comprisethe rays 707 ah, 708 ah, 707 bh and 708 bh and themore-greatly-deflected vertically polarized rays comprise the rays 707bv, 708 bv, 707 av and 708 av. Both the horizontally polarized and thevertically polarized rays pass through and are focused by lens 712,which focuses them onto upper mirror 714 a and lower mirror 714 b,respectively.

FIG. 7c provides a perspective view of the focused wavelengths of thevarious channels of composite optical signal 701 upon the mirrors 714 aand 714 b of the first preferred embodiment of the compensator inaccordance with the present invention. Preferably, each of these mirrorsis shaped as a curved conical mirror. Chromatic dispersion is adjustedby simultaneous movement of both mirrors 714 a- 714 b along theadjustment direction 726 and PMD dispersion is adjusted by movement ofone of the mirrors 714 a- 714 b along the adjustment direction 728.

The mirrors 714 a- 714 b reflect the wavelengths of the channelscomprising the composite optical signal 701 back through the variouscomponents of the compensator 700 so as to be recombined into channelsat birefringent wedge 720 and into a dispersion compensated compositeoptical signal at transmission grating 710 and VIPA 706. Compensatorydispersion is introduced into the return signal by virtue of the factthat different wavelengths “return” to different virtual images of thebeam waist 705 within VIPA 706 as previously described. The degree ofchromatic dispersion introduced into any channel is determined by thecurvature of the mirrors at the locations where the channel isreflected. The degree of introduced chromatic dispersion slope isdetermined by the change in curvature of the mirrors along adjustmentdirection 726. The degree of PMD introduced into any channel isdetermined by the difference in reflection angle between thehorizontally polarized rays as reflected off mirror 714 a and thevertically polarized rays as reflected off mirror 714 b. This differenceis controlled by motion of one of the mirrors 714 a- 714 b along theadjustment direction 728.

FIGS. 8a- 8 b illustrate a top-view and side-view, respectively, of asecond preferred embodiment of a dispersion compensator in accordancewith the present invention. The compensator 800 is similar inconstruction and operation to the compensator 700 except that the singlebirefringent wedge 720 and the single focusing lens 712 comprisingcompensator 700 are respectively replaced by the two birefringent wedges820 a- 820 b and the two focusing lenses 812 a-812 b in the compensator800. One of ordinary skill in the art will recognize that the twobirefringent wedges 820 a- 820 b may be replaced by a singlebirefringent plate. The configuration of elements in the compensator 800permits each lens/mirror assembly 824 a- 824 b to be adjustedindependently of the other along adjustment direction 830. Thisadjustment along the adjustment direction 830 provides an additionalmeans of compensating PMD by causing a difference between the physicalpath lengths of horizontally polarized rays 807 ah, 807 bh, 808 ah, and808 bh and of vertically polarized rays 807 av, 807 bv, 808 av and 808bv. PMD compensation may also be varied in the compensator 800 byadjusting one or more of the mirrors 714 a- 714 b according toadjustment direction 728.

FIGS. 9a- 9 b illustrate a top view and side view, respectively, of athird preferred embodiment of a dispersion compensator in accordancewith the present invention. Like the compensator 700, the compensator900 simultaneously introduces dispersion into optical channels so as tocompensate for chromatic dispersion, chromatic dispersion slope and PMD.However, the compensator 900 differs from the compensator 700 throughthe substitution of two parallel transmission gratings 910 a- 910 b forthe single transmission grating 710 and the optional addition of a beamcondenser 916 comprising two lenses 917 a- 917 b.

The parallel transmission gratings 910 a- 910 b of compensator 900 causethe dispersed wavelengths of all channels to propagate parallel to oneanother in top-view projection after emerging from the grating pair. Thewavelengths 907 a and 907 b represent relatively longer and shorterwavelengths of a first such channel; the wavelengths 908 a, and 908 brepresent relatively longer and shorter wavelengths of a second suchchannel. Because of the parallelism of the gratings 910 a- 910 b, thetwo mirrors 914 a- 914 b do not necessarily require the curved conicalshape of the mirrors 714 a- 714 b. In the third preferred embodiment,the mirrors 914 a- 914 b comprise simple conical shapes, althoughnumerous other shapes are possible. Optionally, the separations betweenthe channels are then condensed along a horizontal dimensionperpendicular to the main axis by the two cylindrical lenses 917 a-917 bcomprising beam condenser 916. Thus, the degree of chromatic dispersionslope compensation may be controlled by adjustment of the beam condenser916. The horizontally polarized rays 907 ah, 907 bh, 908 ah, 908 bh andthe vertically polarized rays 907 av, 907 bv, 908 av, 908 bv of tworepresentative channels are then focused by lens 912 onto first mirror914 a and second mirror 914 b, respectively. In other aspects, theoperation of the compensator 900 is similar to that of the compensator700.

FIGS. 10a- 10 b illustrate a top view and side view, respectively, of afourth preferred embodiment of a dispersion compensator in accordancewith the present invention. The compensator 1000 is similar inconstruction and operation to the compensator 900 except that the singlebirefringent wedge 920 and the single focusing lens 912 comprisingcompensator 900 are respectively replaced by the two birefringent wedges1020 1- 1020 b and the two focusing lenses 1012 a- 1012 b in thecompensator 1000. The configuration of elements in the compensator 1000permits each lens/mirror assembly 1024 a- 1024 b to be adjustedindependently of the other along adjustment direction 1032. Thisadjustment along the adjustment direction 1032 provides an additionalmeans of compensating PMD by causing a difference between the physicalpath lengths of horizontally polarized rays 1007 ah, 1007 bh, 1008 ah,and 1108 bh and of vertically polarized rays 1007 av, 1007 bv, 1008 avand 1008 bv. As in the compensator 900, PMD compensation may also bevaried in the compensator 1000 by adjusting one or more of the mirrors1014 a- 1014 b according to adjustment direction 928.

FIG. 11 illustrates a preferred embodiment of a system which utilizesthe dispersion compensator in accordance with the present invention. Thesystem 1100 comprises an input fiber optic line 1102, an optical tap1111, a dispersion analyzer 1108, a compensator controller 1110, adispersion compensator 1112, a polarization controller 1114, an opticalcirculator 1106 and an output fiber optic line 1104. The input line 1102and output line 1104 are optically coupled to the port 1151 and to theport 1153 of circulator 1106, respectively. The system 1100 furthercomprises a fiber optic tap line 1105 optically coupling the optical tap1111 to the dispersion analyzer 1108 and a fiber optic line 1103optically coupling the dispersion compensator 1112 to port 1152 of thecirculator 1106. The system further comprises first 1107 and second 1109electronic signal or control lines respectively connected between thedispersion analyzer 1108 and the controller 1110 and between thecontroller 1110 and the dispersion compensator 1112. The system furthercomprises a third electronic signal or control line 1113 connectedbetween the dispersion analyzer 1108 and the polarization controller1114. The polarization controller 1114 may be one of several well-knowntypes, such as a looped fiber device, an optical wave plate device, oran electronic liquid crystal device. The polarization controller 1114serves to convert or rotate the polarization state of incominguncompensated signal light into a polarization state compatible with thebirefringent elements of the compensator 1112 so as to provide optimalPMD compensation. The fiber optic line 1103 preferably comprises apolarization-preserving fiber.

An uncompensated optical signal or composite optical signal 1101 u isinput to the system 1100 via the input fiber optic line 1102. Theuncompensated signal 1101 u comprises unwanted chromatic dispersion andPMD that is to be compensated by the system 1100. The uncompensatedoptical signal or composite signal 1101 u passes through thepolarization controller 1114 to the port 1151 of the optical circulator1106. The optical circulator 1106 directs signal 1101 u to its port1152, from which it is immediately output to the fiber optic line 1103and input to the dispersion compensator 1112. The dispersion compensator1112 comprises one of the dispersion compensator embodiments inaccordance with the present invention.

As described previously herein, the dispersion compensator 1112 providescompensatory chromatic dispersion, dispersion slope, and PMD to theuncompensated optical signal or composite optical signal 1101 u so as tooutput the compensated signal or composite optical signal 1101 c. Thecompensated signal 1101 c is output along the optical fiber line 1103 inthe opposite direction from the input signal 1101 u. The compensatedsignal is then input to optical circulator 1106 through its port 1152.By the well-known operation of optical circulators, the compensatedsignal 1101 c is directed to the port 1153 of optical circulator 1106,from which it is immediately output to the output fiber optic line 1104.A small portion 1101 s of the compensated output signal 1101 c is splitoff from signal 1101 c by the optical tap 1111 and diverted to thedispersion analyzer via the fiber optic tap line 1105.

The dispersion compensator 1112 is controlled by electronic signal 1118output from controller 1110 along electronic line 1109. The controller1110 generates control signals in response to an electronic signal orsignals 1116 produced by dispersion analyzer 1108 and sent to thecontroller 1110 along electronic line 1107. The dispersion analyzer maycomprise separate known components to analyze chromatic dispersion andPMD, such as an optical spectrum analyzer and an ellipsometer,respectively. The electronic signal(s) 1116 contains informationmeasured by the dispersion analyzer 1108 and pertaining to the magnitudeand sign of chromatic dispersion and PMD carried by the sample signal1101 s. By logical extension, these quantities also relate to the signal1101 u. In response to these measurements, the dispersion analyzer 1108outputs a first electronic signal 1116 to controller 1110 alongelectronic line 1107 and, optionally, outputs a second electronic signal1120 to the polarization controller along electronic line 1113.

The amount of compensatory dispersion provided by dispersion compensator1112 is controlled by the electronic signal 1118 output from thecontroller 1110 in response to the dispersion characteristics measuredby dispersion analyzer 1108. Adjusting one or more of the variousoptical components along its respective adjustment direction, asdescribed previously herein, causes variation in the magnitude and signof the compensatory dispersion. It may be necessary to separate and orrotate the polarization components of the uncompensated signal 1101 u,prior to input to the dispersion compensator 1112. The polarizationcontroller 1114 performs these polarization separation and rotationfunctions.

A dispersion compensator which utilizes a Virtually Imaged Phased Array(VIPA), gratings, and birefringent wedges to moderate chromaticdispersion, dispersion slope and polarization mode dispersion has beendisclosed. The dispersion compensator in accordance with the presentinvention provides simultaneous tunable compensation of these variousdispersions utilizing a single apparatus. A system which utilizes thecompensator is thus cost effective to manufacture.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method for dispersion compensation for acomposite optical signal in an optical fiber transmission system, thecomposite optical signal comprising a plurality of channels, each of theplurality of channels comprising a band of wavelengths, wherein thebands of wavelengths comprises unwanted chromatic dispersion, dispersionslope, and polarization mode dispersion (PMD), comprising the steps of:(a) propagating the composite optical signal in a forward direction; (b)separating the wavelengths in the band of wavelengths in each of theplurality of channels, wherein each of the wavelengths in the band isspatially distinguishable from the other wavelengths in the band; (c)spatially separating each band of wavelengths in the plurality ofchannels; (d) spatially separating each wavelength of each separatedband of wavelengths into a plurality of polarized rays; and (e)reflecting the plurality of polarized rays toward a return direction,wherein dispersion is added to the reflected plurality of polarized rayssuch that the unwanted chromatic dispersion, dispersion slope, and PMDare compensated.
 2. The method of claim 1, wherein the spatiallyseparating step (d) comprises: (d1) spatially separating each wavelengthof each band of wavelengths into a first polarized ray and a secondpolarized ray, wherein a polarization plane orientation of the first andsecond polarized rays are mutually orthogonal.
 3. The method of claim 1,wherein in the reflecting step (e), each of the plurality of polarizedrays intercepts at least one mirror at a different position.
 4. A methodfor dispersion compensation for a composite optical signal in an opticalfiber transmission system, the composite optical signal comprising aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the bands of wavelengths comprises unwantedchromatic dispersion, dispersion slope, and polarization mode dispersion(PMD), comprising the steps of: (a) propagating the composite opticalsignal in a forward direction; (b) separating the wavelengths in theband of wavelengths in each of the plurality of channels, wherein eachof the wavelengths in the band is spatially distinguishable from theother wavelengths in the band; (c) spatially separating each band ofwavelengths in the plurality of channels, wherein the spatiallyseparating step (c) further comprises: (c1) compressing the spatiallyseparated bands of wavelengths along a single dimension; (d) spatiallyseparating each wavelength of each separated band of wavelengths into aplurality of polarized rays; and (e) reflecting the plurality ofpolarized rays toward a return direction, wherein dispersion is added tothe reflected plurality of polarized rays such that the unwantedchromatic dispersion, dispersion slope, and PMD are compensated.
 5. Amethod for dispersion compensation for a composite optical signal in anoptical fiber transmission system, the composite optical signalcomprising a plurality of channels, each of the plurality of channelscomprising a band of wavelengths, wherein the bands of wavelengthscomprises unwanted chromatic dispersion, dispersion slope, andpolarization mode dispersion (PMD), comprising the steps of: (a)propagating the composite optical signal in a forward direction; (b)separating the wavelengths in the band of wavelengths in each of theplurality of channels, wherein each of the wavelengths in the band isspatially distinguishable from the other wavelengths in the band; (c)spatially separating each band of wavelengths in the plurality ofchannels; (d) spatially separating each wavelength of each separatedband of wavelengths into a plurality of polarized rays; and (e)reflecting the plurality of polarized rays toward a return direction,wherein dispersion is added to the reflected plurality of polarized rayssuch that the unwanted chromatic dispersion, dispersion slope, and PMDare compensated, wherein in the reflecting step (e), each of theplurality of polarized rays intercepts at least one mirror at adifferent position, wherein a first polarized ray intercepts a firstmirror and a second polarized ray intercepts a second mirror.
 6. Adispersion compensator, comprising: a Virtually Imaged Phased Array(VIPA) optically coupled to an optical fiber transmission system at afirst side, wherein a composite optical signal from the optical fibertransmission system is capable of traversing through the VIPA in aforward direction, wherein the composite optical signal comprises aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the bands of wavelengths comprise unwantedchromatic dispersion, dispersion slope, and PMD; at least onediffraction grating, wherein a first side of the at least onediffraction grating is optically coupled to a second side of the VIPA;at least one birefringent wedge, wherein a first side of the at leastone birefringent wedge is optically coupled to a second side of the atleast one diffraction grating; at least one focusing lens, wherein afirst side of the at least one focusing lens is optically coupled to asecond side of the at least one birefringent wedge; and at least onemirror optically coupled to a second side of the at least one focusinglens, wherein the at least one mirror reflects a plurality of polarizedrays of each of spatially separated bands of wavelengths toward a returndirection, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated.
 7. The compensator of claim 6, whereinthe VIPA separates the wavelengths in a band of wavelengths in each of aplurality of channels, wherein each of the wavelengths in the band isspatially distinguishable from the other wavelengths in the band.
 8. Thecompensator of claim 6, wherein the at least one diffraction gratingspatially separates each band of wavelengths in the plurality ofchannels traversing through the at least one diffraction grating in theforward direction.
 9. The compensator of claim 6, wherein the at leastone birefringent wedge spatially separates each wavelength of each bandof wavelengths traversing through the at least one birefringent wedge inthe forward direction into the plurality of polarized rays.
 10. Thecompensator of claim 9, wherein each wavelength is separated into theplurality of polarized rays within a vertical plane.
 11. The compensatorof claim 9, wherein each wavelength is separated into the plurality ofpolarized rays within a horizontal plane.
 12. The compensator of claim6, wherein the at least one diffraction grating comprises: a firstdiffraction grating, wherein a first side of the first diffractinggrating is optically coupled to the second side of the VIPA; and asecond diffraction grating, wherein a first side of the seconddiffraction grating is optically coupled to a second side of the firstdiffraction grating, wherein a second side of the second diffractiongrating is optically coupled to the first side of the at least onebirefringent wedge.
 13. The compensator of claim 6, wherein the at leastone birefringent wedge comprises: a first birefringent wedge, wherein afirst side of the first birefringent wedge is optically coupled to thesecond side of the at least one diffraction grating; and a secondbirefringent wedge, wherein a first side of the second birefringentwedge is optically coupled to a second side of the first birefringentwedge, wherein a second side of the second birefringent wedge isoptically coupled to the first side of the at least one focusing lens.14. The compensator of claim 13, further comprising: a beam condenseroptically coupled between the first birefringent wedge and the secondbirefringent wedge.
 15. The compensator of claim 6, wherein the at leastone focusing lens comprises: a first focusing lens, wherein a first sideof the first focusing lens is optically coupled to the second side ofthe at least one birefringent wedge and a second side of the firstfocusing lens is optically coupled to the at least one mirror; and asecond focusing lens, wherein a first side of the second focusing lensis optically coupled to the second side of the at least one birefringentwedge and a second side of the second focusing lens is optically coupledto the at least one mirror.
 16. The compensator of claim 6, wherein theat least one mirror comprises a flat surface.
 17. The compensator ofclaim 6, wherein the at least one mirror comprises a cylindrical concavesurface.
 18. The compensator of claim 6, wherein the at least one mirrorcomprises a cylindrical convex surface.
 19. The compensator of claim 6,wherein the at least one mirror is adjustable in a vertical direction,wherein a vertical adjustment varies an amount of dispersion added. 20.The compensator of claim 6, wherein the at least one mirror isadjustable in a horizontal direction, wherein a horizontal adjustmentvaries an amount of dispersion added.
 21. The compensator of claim 6,wherein the at least one mirror is rotationally adjustable, wherein arotational adjustment varies an amount of dispersion added.
 22. Thecompensator of claim 6, further comprising: a beam condenser opticallycoupled between the at least one birefringent wedge and the at least onefocusing lens.
 23. A dispersion compensator, comprising: a VirtuallyImaged Phased Array (VIPA) optically coupled to an optical fibertransmission system at a first side, wherein a composite optical signalfrom the optical fiber transmission system is capable of traversingthrough the VIPA in a forward direction, wherein the composite opticalsignal comprises a plurality of channels, each of the plurality ofchannels comprising a band of wavelengths, wherein the bands ofwavelengths comprise unwanted chromatic dispersion, dispersion slope,and PMD; at least one diffraction grating, wherein a first side of theat least one diffraction grating is optically coupled to a second sideof the VIPA; at least one birefringent wedge, wherein a first side ofthe at least one birefringent wedge is optically coupled to a secondside of the at least one diffraction grating; at least one focusinglens, wherein a first side of the at least one focusing lens isoptically coupled to a second side of the at least one birefringentwedge; and at least one mirror optically coupled to a second side of theat least one focusing lens, wherein the at least one mirror reflects aplurality of polarized rays of each of spatially separated bands ofwavelengths toward a return direction, wherein dispersion is added tothe reflected plurality of polarized rays such that the unwantedchromatic dispersion, dispersion slope, and PMD are compensated, whereinthe at least one mirror comprises: a first mirror optically coupled tothe second side of the at least one focusing lens, and a second mirroroptically coupled to the second side of the at least one focusing lens.24. The compensator of claim 23, wherein the first mirror reflects afirst polarized ray and the second mirror reflects a second polarizedray, wherein a polarization plane orientation of the first and secondpolarized rays are mutually orthogonal.
 25. A dispersion compensator,comprising: a VIPA optically coupled to an optical fiber transmissionsystem at a first side, wherein a composite optical signal from theoptical fiber transmission system is capable of traversing through theVIPA in a forward direction, wherein the composite optical signalcomprises a plurality of channels, each of the plurality of channelscomprising a band of wavelengths, wherein the bands of wavelengthscomprise unwanted chromatic dispersion, dispersion slope, and PMD; adiffraction grating, wherein a first side of the diffraction grating isoptically coupled to a second side of the VIPA; a first birefingentwedge, wherein a first side of the first birefringent wedge is opticallycoupled to a second side of the diffraction grating; a secondbirefringent wedge, wherein a first side of the second birefringentwedge is optically coupled to a second side of the first birefringentwedge; a first focusing lens, wherein a first side of the first focusinglens is optically coupled to a second side of the second birefringentwedge; a second focusing lens, wherein a first side of the secondfocusing lens is optically coupled to the second side of the secondbirefringent wedge; a first mirror optically coupled to a second side ofthe first focusing lens, wherein the first mirror reflects a firstpolarized ray of each wavelength of each spatially separated band ofwavelengths toward a return direction; and a second mirror opticallycoupled to a second side of the second focusing lens, wherein the secondmirror reflects a second polarized ray of each wavelength of eachspatially separated band of wavelengths toward a return direction,wherein a polarization plane orientation of the first and secondpolarized rays are mutually orthogonal, wherein dispersion is added tothe reflected plurality of polarized rays such that the unwantedchromatic dispersion, dispersion slope, and PMD are compensated.
 26. Adispersion compensator, comprising: a VIPA optically coupled to anoptical fiber transmission system at a first side, wherein a compositeoptical signal from the optical fiber transmission system is capable oftraversing through the VIPA in a forward direction, wherein thecomposite optical signal comprises a plurality of channels, each of theplurality of channels comprising a band of wavelengths, wherein thebands of wavelengths comprise unwanted chromatic dispersion, dispersionslope, and PMD; a first diffraction grating, wherein a first side of thefirst diffraction grating is optically coupled to a second side of theVIPA; a second diffraction grating, wherein a first side of the seconddiffraction grating is optically coupled to a second side of the firstdiffraction grating; a birefringent wedge, wherein a first side of thebirefringent wedge is optically coupled to a second side of the seconddiffraction grating; a focusing lens, wherein a first side of thefocusing lens is optically coupled to a second side of the birefringentwedge; a first mirror optically coupled to a second side of the focusinglens, wherein the first mirror reflects a first polarized ray of eachwavelength of each spatially separated band of wavelengths toward areturn direction; and a second mirror optically coupled to the secondside of the focusing lens, wherein the second mirror reflects a secondpolarized ray of each wavelength of each spatially separated band ofwavelengths toward a return direction, wherein a polarization planeorientation of the first and second polarized rays are mutuallyorthogonal, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated.
 27. The compensator of claim 26, furthercomprising: a beam condenser optically coupled between the birefringentwedge and the focusing lens.
 28. A dispersion compensator, comprising: aVIPA optically coupled to an optical fiber transmission system at afirst side, wherein a composite optical signal from the optical fibertransmission system is capable of traversing through the VIPA in aforward direction, wherein the composite optical signal comprises aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the bands of wavelengths comprise unwantedchromatic dispersion, dispersion slope, and PMD; a first diffractiongrating, wherein a first side of the first diffraction grating isoptically coupled to a second side of the VIPA; a second diffractiongrating, wherein a first side of the second diffraction grating isoptically coupled to a second side of the first diffraction grating; afirst birefringent wedge, wherein a first side of the first birefringentwedge is optically coupled to a second side of the second diffractiongrating; a second birefringent wedge, wherein a first side of the secondbirefringent wedge is optically coupled to a second side of the firstbirefringent wedge; a first focusing lens, wherein a first side of thefirst focusing lens is optically coupled to a second side of the secondbirefringent wedge; a second focusing lens, wherein a first side of thesecond focusing lens is optically coupled to the second side of thesecond birefringent wedge; a first mirror optically coupled to a secondside of the first focusing lens, wherein the first mirror reflects afirst polarized ray of each wavelength of each spatially separated bandof wavelengths toward a return direction; and a second mirror opticallycoupled to a second side of the second focusing lens, wherein the secondmirror reflects a second polarized ray of each wavelength of eachspatially separated band of wavelengths toward a return direction,wherein a polarization plane orientation of the first and secondpolarized rays are mutually orthogonal, wherein dispersion is added tothe reflected plurality of polarized rays such that the unwantedchromatic dispersion, dispersion slope, and PMD are compensated.
 29. Thecompensator of claim 28, further comprising: a beam condenser opticallycoupled between the first birefringent wedge and the second birefringentwedge.
 30. A system, comprising: an optical fiber transmission system;and a dispersion compensator optically coupled to the optical fibertransmission system, the dispersion compensator comprising: a VIPAoptically coupled to the optical fiber transmission system at a firstside, wherein a composite optical signal from the optical fibertransmission system is capable of traversing through the VIPA in aforward direction, wherein the composite optical signal comprises aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the bands of wavelengths comprise unwantedchromatic dispersion, dispersion slope, and PMD; at least onediffraction grating, wherein a first side of the at least onediffraction grating is optically coupled to a second side of the VIPA;at least one birefringent wedge, wherein a first side of the at leastone birefringent wedge is optically coupled to a second side of the atleast one diffraction grating; at least one focusing lens, wherein afirst side of the at least one focusing lens is optically coupled to asecond side of the at least one birefringent wedge; and at least onemirror optically coupled to a second side of the at least one focusinglens, wherein the at least one mirror reflects a plurality of polarizedrays of each of spatially separated bands of wavelengths toward a returndirection, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated.
 31. The system of claim 30, wherein theVIPA separates the wavelengths in a band of wavelengths in each of aplurality of channels, wherein each of the wavelengths in the band isspatially distinguishable from the other wavelengths in the band. 32.The system of claim 30, wherein the at least one diffraction gratingspatially separates each band of wavelengths in the plurality ofchannels traversing through the at least one diffraction grating in theforward direction.
 33. The system of claim 30, wherein the at least onebirefringent wedge spatially separates each wavelength of each band ofwavelengths traversing through the at least one birefringent wedge inthe forward direction into the plurality of polarized rays.
 34. Thesystem of claim 33, wherein each wavelength is separated into theplurality of polarized rays within a vertical plane.
 35. The system ofclaim 33, wherein each wavelength is separated into the plurality ofpolarized rays within a horizontal plane.
 36. The system of claim 30,wherein the at least one diffraction grating comprises: a firstdiffraction grating, wherein a first side of the first diffractinggrating is optically coupled to the second side of the VIPA; and asecond diffraction grating, wherein a first side of the seconddiffraction grating is optically coupled to a second side of the firstdiffraction grating, wherein a second side of the second diffractiongrating is optically coupled to the first side of the at least onebirefringent wedge.
 37. The system of claim 30, wherein the at least onebirefringent wedge comprises: a first birefringent wedge, wherein afirst side of the first birefringent wedge is optically coupled to thesecond side of the at least one diffraction grating; and a secondbirefringent wedge, wherein a first side of the second birefringentwedge is optically coupled to a second side of the first birefringentwedge, wherein a second side of the second birefringent wedge isoptically coupled to the first side of the at least one focusing lens.38. The system of claim 37, further comprising: a beam condenseroptically coupled between the first birefringent wedge and the secondbirefringent wedge.
 39. The system of claim 30, wherein the at least onefocusing lens comprises: a first focusing lens, wherein a first side ofthe first focusing lens is optically coupled to the second side of theat least one birefringent wedge and a second side of the first focusinglens is optically coupled to the at least one mirror; and a secondfocusing lens, wherein a first side of the second focusing lens isoptically coupled to the second side of the at least one birefringentwedge and a second side of the second focusing lens is optically coupledto the at least one mirror.
 40. The system of claim 30, wherein the atleast one mirror comprises a flat surface.
 41. The system of claim 30,wherein the at least one mirror comprises a cylindrical concave surface.42. The system of claim 30, wherein the at least one mirror comprises acylindrical convex surface.
 43. The system of claim 30, wherein the atleast one mirror is adjustable in a vertical direction, wherein avertical adjustment varies an amount of dispersion added.
 44. The systemof claim 30, wherein the at least one mirror is adjustable in ahorizontal direction, wherein a horizontal adjustment varies an amountof dispersion added.
 45. The system of claim 30, wherein the at leastone mirror is rotationally adjustable, wherein a rotational adjustmentvaries an amount of dispersion added.
 46. The system of claim 30,further comprising: a beam condenser optically coupled between the atleast one birefringent wedge and the at least one focusing lens.
 47. Asystem, comprising: an optical fiber transmission system; and adispersion compensator optically coupled to the optical fibertransmission system, the dispersion compensator comprising: a VIPAoptically coupled to the optical fiber transmission system at a firstside, wherein a composite optical signal from the optical fibertransmission system is capable of traversing through the VIPA in aforward direction, wherein the composite optical signal comprises aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the bands of wavelengths comprise unwantedchromatic dispersion, dispersion slope, and PMD; at least onediffraction grating, wherein a first side of the at least onediffraction grating is optically coupled to a second side of the VIPA;at least one birefringent wedge, wherein a first side of the at leastone birefringent wedge is optically coupled to a second side of the atleast one diffraction grating; at least one focusing lens, wherein afirst side of the at least one focusing lens is optically coupled to asecond side of the at least one birefringent wedge; and at least onemirror optically coupled to a second side of the at least one focusinglens, wherein the at least one mirror reflects a plurality of polarizedrays of each of spatially separated bands of wavelengths toward a returndirection, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated, wherein the at least one mirrorcomprises: a first mirror optically coupled to the second side of the atleast one focusing lens, and a second mirror optically coupled to thesecond side of the at least one focusing lens.
 48. The system of claim47, wherein the first mirror reflects a first polarized ray and thesecond mirror reflects a second polarized ray, wherein a polarizationplane orientation of the first and second polarized rays are mutuallyorthogonal.
 49. A system comprising: an optical fiber transmissionsystem; an optical circulator, wherein a first port and a third port ofthe optical circulator are optically coupled to the optical fibertransmission system; and a dispersion compensator optically coupled to asecond port of the optical circulator, the dispersion compensatorcomprising: a VIPA optically coupled to the optical fiber transmissionsystem at a first side, wherein a composite optical signal from theoptical fiber transmission system is capable of traversing through theVIPA in a forward direction, wherein the composite optical signalcomprises a plurality of channels, each of the plurality of channelscomprising a band of wavelengths, wherein the bands of wavelengthscomprise unwanted chromatic dispersion, dispersion slope, and PMD; atleast one diffraction grating, wherein a first side of the at least onediffraction grating is optically coupled to a second side of the VIPA;at least one birefringent wedge, wherein a first side of the at leastone birefringent wedge is optically coupled to a second side of the atleast one diffraction grating; at least one focusing lens, wherein afirst side of the at least one focusing lens is optically coupled to asecond side of the at least one birefringent wedge; and at least onemirror optically coupled to a second side of the at least one focusinglens, wherein the at least one mirror reflects a plurality of polarizedrays of each of spatially separated bands of wavelengths toward a returndirection, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated.
 50. A system, comprising: a dispersioncompensator; and a composite optical signal traversing through thedispersion compensator, wherein the composite signal comprises aplurality of channels, each of the plurality of channels comprising aband of wavelengths, wherein the band of wavelengths comprises unwantedchromatic dispersion, dispersion slope and PMD, wherein the compositeoptical signal is propagated in a forward direction, wherein thewavelengths in the band of wavelengths in each of the plurality ofchannels are separated, wherein each of the wavelengths in the band isspatially distinguishable from the other wavelengths in the band,wherein each band of wavelengths in the plurality of channels isspatially separated, wherein each wavelength of each separated band ofwavelengths is spatially separated into a plurality of polarized rays,and wherein the plurality of polarized rays is reflected toward a returndirection, wherein dispersion is added to the reflected plurality ofpolarized rays such that the unwanted chromatic dispersion, dispersionslope, and PMD are compensated.
 51. A system, comprising: means forpropagating a composite optical signal in a forward direction, whereinthe composite optical signal comprises a plurality of channels, each ofthe plurality of channels comprising a band of wavelengths, wherein theband of wavelengths comprises unwanted chromatic dispersion, dispersionslope and PMD; means for separating the wavelengths in the band ofwavelengths in each of the plurality of channels, wherein each of thewavelengths in the band is spatially distinguishable from the otherwavelengths in the band; means for spatially separating each band ofwavelengths in the plurality of channels; means for spatially separatingeach wavelength of each separated band of wavelengths into a pluralityof polarized rays; and means for reflecting the plurality of polarizedrays toward a return direction, wherein dispersion is added to thereflected plurality of polarized rays such that the unwanted chromaticdispersion, dispersion slope, and PMD are compensated.